Pandas version checks
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[X] I have checked that this issue has not already been reported.
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[X] I have confirmed this bug exists on the latest version of pandas.
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[ ] I have confirmed this bug exists on the main branch of pandas.
Reproducible Example
>>> df = pd.DataFrame({'a': [1,2,3, float('nan')], 'b':[float('nan'), 3,3,3]})
>>> df[['a', 'b']].fillna('', inplace=True)
>>> df
a b
0 1.0 NaN
1 2.0 3.0
2 3.0 3.0
3 NaN 3.0
>>> df.fillna('', inplace=True)
>>> df
a b
0 1.0
1 2.0 3.0
2 3.0 3.0
3 3.0
>>> df = pd.DataFrame({'a': [1,2,3, float('nan')], 'b':[float('nan'), 3,3,3]})
>>> df
a b
0 1.0 NaN
1 2.0 3.0
2 3.0 3.0
3 NaN 3.0
>>> df['a'].fillna('', inplace=True)
>>> df
a b
0 1.0 NaN
1 2.0 3.0
2 3.0 3.0
3 3.0
Issue Description
[] indexing does not seem to work for DataFrame.fillna when inplace is True. [] indexing seems to work for Series.fillna when inplace is True
Expected Behavior
df = pd.DataFrame({'a': [1,2,3, float('nan')], 'b':[float('nan'), 3,3,3]}) df[['a', 'b']].fillna('', inplace=True) df a b 0 1.0 1 2.0 3.0 2 3.0 3.0 3 3.0
Installed Versions
>>> pd.show_versions()
INSTALLED VERSIONS
------------------
commit : 91111fd99898d9dcaa6bf6bedb662db4108da6e6
python : 3.8.10.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19044
machine : AMD64
processor : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : Swedish_Sweden.1252
pandas : 1.5.1
numpy : 1.20.3
pytz : 2021.1
dateutil : 2.8.1
setuptools : 56.0.0
pip : 22.2.2
Cython : None
pytest : 6.2.4
hypothesis : None
sphinx : 4.2.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.6.3
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.1
IPython : 7.25.0
pandas_datareader: None
bs4 : 4.10.0
bottleneck : None
brotli : None
fastparquet : None
fsspec : 2021.10.1
gcsfs : None
matplotlib : 3.4.2
numba : None
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.7
pandas_gbq : None
pyarrow : 6.0.0
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.7.1
snappy : None
sqlalchemy : 1.4.26
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
zstandard : None
tzdata : None
Comment From: phofl
hi, thanks for your report. This behaves as expected. Indexing with [] creates a copy, hence you are not modifying the original object.